Flat-panel display devices are widely used in conjunction with computing devices, in portable devices, and for entertainment devices such as televisions. Such displays typically employ a plurality of pixels distributed over a substrate to display images. Display devices are typically controlled with either a passive-matrix control employing electronic circuitry external to the substrate or an active-matrix control employing electronic circuitry formed directly on the substrate. Organic light emitting diode (OLED) display devices have been fabricated with active-matrix (AM) driving circuitry in order to produce high-performance displays. An example of such an AM OLED display device is disclosed in U.S. Pat. No. 5,550,066. Active-matrix circuitry is commonly achieved by forming thin-film transistors (TFTs) over a substrate and employing a separate circuit to control each light-emitting pixel in the display.
In an active-matrix device, active control elements include thin-films of semiconductor material formed over a substrate, for example amorphous or poly-crystalline silicon, and distributed over a flat-panel display substrate. Typically, each display sub-pixel is controlled by one control element and each control element includes at least one transistor. For example, in a simple active-matrix organic light-emitting display, each control element includes two transistors (a select transistor and a power transistor) and one capacitor for storing a charge specifying the luminance of the sub-pixel. Each light-emitting element typically employs an independent control electrode and a common electrode. Control of the light-emitting elements is typically provided through a data signal line, a select signal line, a power connection and a ground connection formed as metal wires over the substrate. Active-matrix elements are not necessarily limited to displays and can be distributed over a substrate and employed in other applications requiring spatially distributed control.
Thin-film transistors (TFTs) are composed of a thin layer (usually 100-400 nm) of a semiconductor such as amorphous silicon or polysilicon. The properties of such thin-film semiconductors are, however, often not sufficient for constructing a high-quality display. Amorphous silicon, for example, is unstable in that its threshold voltage (Vth) and carrier mobility shifts over extended periods of use. Polysilicon often has a large degree of variability across the substrate in threshold voltage (Vth) and carrier mobility due to the crystallization process. Since OLED devices operate by current injection, variability in the TFTs can result in variability of the luminance of the OLED pixels and degrade the visual quality of the display. Novel compensation schemes, such as adding additional TFT circuitry in each pixel, have been proposed to compensate for TFT variability, however, such compensation adds complexity which can negatively impact yield, cost, or reduce the OLED emission area. Furthermore, as thin-film transistor fabrication processes are applied to larger substrates such as used for large flat-panel television applications, the variability and process cost increase.
One approach to avoid these issues with thin-film transistors is instead to fabricate conventional transistors in a semiconductor substrate and then transfer these transistors onto a display substrate. U.S. Patent Application Publication No. 2006/0055864 A1 by Matsumura et al. teaches a method for the assembly of a display using semiconductor integrated circuits (ICs) affixed within the display for controlling pixel elements where the embedded transistors in the ICs replace the normal functions performed by the TFTs of prior-art displays. Matsumura teaches that the semiconductor substrate should be thinned, for example by polishing, to a thickness of between 20 micrometers to 100 micrometers. The substrate is then diced into smaller pieces containing the integrated circuits, hereafter referred to as ‘chiplets’. Matsumura teaches a method of cutting the semiconductor substrate, for example by etching, sandblasting, laser-beam machining, or dicing. Matsumura also teaches a pick-up method where the chiplets are selectively picked up using a vacuum chuck system with vacuum holes corresponding to a desired pitch. The chiplets are then transferred to a display substrate where they are embedded in a thick thermoplastic resin. Wiring interconnections within the pixel-control device and connections from busses and control electrodes to the pixel-control device are shown. In order for wiring interconnections to be successfully made to the chiplets, it is necessary to locate the chiplets with a high degree of accuracy and reliability. If the substrate is contaminated or improperly prepared, chiplets cannot adhere or align adequately, thus preventing the substrate from operating properly. In particular, chiplets cannot be present in some locations or can be present but not properly positioned.
There is a need therefore, for a manufacturing process that corrects substrates for missing or improperly aligned chiplets.